Mast, Light Reactions in Lower Organisms. 173 



of illumination which he used. Shibata ('05, p. 573) repeated 

 Pfeffer's experiments on the reactions of fern spermatozoids to 

 malic acid, using the capillary tube method. He also ascertained 

 the threshold for this organism when stimulated by potassium 

 fumarate, succinate, or tartarate, in various degrees of concentra- 

 tion. He found the reactions to all of these chemical compounds 

 to take place in accordance w^ith the law of Weber. 



A number of other investigators have worked on this subject, 

 but thus far no one has tested the validity of the law for the light 

 reactions in motile organisms. In other words, no one has ascer- 

 tained the minimum difference in light intensity on opposite sides 

 which will cause a response of motile organisms in different degrees 

 of total illumination. The following experiments were under- 

 taken for the purpose of getting evidence concerning this matter. 



The use and arrangement of apparatus used in these experi- 

 ments will readily be understood by referring to the accompany- 

 ing diagram. 



The box, containing a small opening in front of which the Nernst 

 glower was mounted, served as a non-reflecting background. The 

 screens surrounding the glower were so constructed and arranged 

 that no light escaped excepting that which passed through the 

 opening represented in Fig. 15. This light was absorbed after 

 being used to illuminate the aquarium, and since no other light 

 entered the room in which the experiments were performed, it is 

 clear that the reactions observed, and recorded in the following 

 tables, were induced only by light directly from the glower. 



The glass tube, represented in the center of the aquarium. Fig. 

 15, by a ring, extended about 2 cm. above the upper edges of the 

 glass walls and was so fastened that it could be easily raised verti- 

 cally. In each exposure enough filtered water was put into the 

 aquarium to fill it to a point a few millimeters above the upper 

 edge of the opening in the screen s, on either side of the aquarium. 

 The glass tube was then put in place and a number of colonies 

 introduced. The tube formed such close connections with the 

 bottom of the aquarium that the colonies could not get out, but 

 the water introduced with them could. As soon as a state of 

 equilibrium was established, the colonies were set free by carefully 

 raising the tube. After they had been exposed to the light from 

 opposite directions for a few moments, the contents of the aqua- 

 rium was divided into two equal parts by means of a piece of tin 



